In wireless communications, the term “multiple-input multiple-output,” or MIMO, refers to methods in which the communicating parties use multiple antennas to better exploit the radio channel. MIMO systems that support communicating with multiple users simultaneously are referred to as multi-user (MU)-MIMO systems.
IEEE standard, “IEEE Std 802.11ac™-2013,” December 2013, whose disclosure is incorporated herein by reference, is a wireless networking standard in the 802.11 family, also known as “Wi-Fi”, which can be used in implementing high-throughput MU-MIMO wireless local area networks (WLANs) on the 5 GHz band.
In some types of MIMO systems, transmissions via multiple antennas are shaped spatially, or precoded, using beamforming techniques, which are typically based on knowledge of the channel between the transmitter and receiver. In some MIMO systems, the transmitter and receiver jointly measure the communication channel using a channel calibration process. The IEEE 802.11ac standard cited above specifies, e.g., in section 9.31, a calibration process of this sort, also referred to as “channel sounding.”